The invention relates generally to positioning systems. More specifically, the invention relates to positioning systems that integrate global positioning systems with indoor positioning systems.
Global positioning systems (GPS) are able to determine the relative position of an antenna coupled to a GPS receiver on or above the surface of the Earth. Twenty-four satellites placed in orbit around the Earth continuously transmit GPS signals on an L-band carrier (L1) frequency of 1.575 GHz. The L1 carrier component is modulated by a course acquisition (C/A) pseudo random noise (PRN) code component and a data component. The PRN code component provides timing information for determining when a satellite transmitted the GPS signal. The data component includes information such as the orbital position of the transmitting satellite. From the timing and satellite orbital position information in the PRN code and data components obtained from the GPS signals of at least four GPS satellites, the GPS receiver performs ranging measurements. By receiving four different GPS signals, the GPS receiver can determine the position of the antenna within meters.
To improve upon the accuracy of GPS, industry developed Differential GPS (DGPS), which is a variant of GPS that uses a fixed ground reference GPS receiver with a precisely known location to relay GPS corrections to a mobile GPS receiver. A general principle of DGPS is that satellite ranging errors affect the reference GPS receiver and mobile GPS receiver alike. Just like the mobile GPS receiver, the reference GPS receiver computes its position from the received GPS signals. The reference GPS receiver then detects ranging errors by comparing the computed position to the known position and computes corrections to these ranging errors for real-time transmission to the mobile GPS receiver. The reference GPS receiver can transmit the corrections using a tradition digital data link to the mobile GPS receiver. Consequently, the mobile GPS receiver applies the corrections to the measurements made from GPS signals received from the same set of satellites as the reference receiver to improve upon its own computed position measurement.
DGPS systems operate best when the antenna of the GPS receiver is outdoors to receive the GPS signals from the satellites (or pseudolites). GPS signals typically fade considerably or completely when penetrating the walls of structures, e.g., buildings. Consequently, a DGPS ceases to track an object bearing the antenna once that object enters indoors. Thus, there remains a need for a comprehensive positioning or tracking system that can track objects both indoors and outdoors.
An object of the invention is to integrate indoor tracking with outdoor tracking. Another object is to enable the tracking of multiple objects simultaneously without producing false position determinations. It is a further object of the invention to enable the tracking system to be portable.
One aspect of the invention features a system for determining a position of an object. The system includes a first communication unit that has a transmitter. The transmitter aperiodically emits an ultrasonic signal. A second communication unit has a receiver to receive the ultrasonic signal and a transmitter to transmit an identification signal in response to receiving the ultrasonic signal. One of the two communication units is placed at a predetermined position and the other of the two communication units is placed with an object. A processor uses identification information obtained from the identification signal to produce position information indicating that the object is at the predetermined position. In one embodiment, the first communication unit is placed at the predetermined position and the second communication unit accompanies the object. In another embodiment, the first communication unit accompanies the object and the second communication unit is placed at the predetermined position.
In one embodiment, the system includes a timer that produces a window of time and a receiver in communication with the timer. The receiver accepts any identification signal that arrives within the window of time and ignores any identification signal that arrives outside of the window of time. The timer opens the window of time a predetermined period of time after the transmitter of the first communication unit emits the ultrasonic signal. As an example, the predetermined period of time approximates a period of time required for the ultrasonic signal to propagate to the second communication unit.
The information conveyed by the identification signal depends upon the particular embodiment. In one embodiment, the identification signal includes information that leads to identifying the predetermined position. For example, the identification signal in this embodiment includes the serial number of the communication unit that transmits the identification signal. A control station maintains a table that cross-references serial numbers of communication units to particular positions where each communication unit is located.
In another embodiment, the identification signal includes information that leads to identifying the object at the predetermined position. Again, the identification signal includes the serial number of the communication unit that transmits the identification signal. A control station maintains a table that cross-references serial numbers of communication units to particular objects accompanying that communication unit.
In another embodiment, the system includes a differential global positioning system (DGPS) that is capable of computing a position of the object from GPS signals received from remote signal transmitters.
In another aspect, the invention features a method for tracking an object. A first communication unit is placed at a predetermined position and a second communication unit accompanies the object. An ultrasonic signal is aperiodically emitted from one of the communication units. A window of time having a limited duration is opened for receiving a response to the ultrasonic signal from the other of the communication units. The response to the ultrasonic signal is accepted if the response arrives within the opened window of time. The predetermined position is associated with the object based on information provided with the response.
In one embodiment, position information of the object is computed from GPS signals received from a plurality of remote signal transmitters using a differential global positioning system (DGPS). In another embodiment, a frequency is assigned to one of the communication units for communicating with a base station and a time slot is allocated within the assigned frequency during which that one communication unit can transmit position information to the base station. In still another embodiment, the ultrasonic signal is detected at more than one position, and an approximate position of the object is calculated based in part on when the ultrasonic signal was detected at each position.
In another aspect, the invention features a system for tracking a position of an object that includes a global positioning system that computes position information about the object when the global positioning system is able to receive signals from a signal transmitter. An indoor tracking system that includes a communication unit placed at a predetermined position, determines position information about the object when the object enters within a predetermined range of the communication unit. A processor receives the position information about the object for transmission to a base station.
In another aspect, the invention features a portable apparatus that accompanies an object and is used for determining a position of the object. A transceiver is placed at a predetermined location. The apparatus includes a global positioning system (GPS) receiver that computes the position of the object from signals concurrently received from a plurality of signal transmitters. A communication unit exchanges ultrasonic and radio frequency signals with a transceiver to determine the position of the object when the object is within range of the transceiver. This transceiver is placed at a predetermined location. A processor receives position information determined by either or both of the GPS receiver and the communication unit.